1. Field of the Invention
This invention relates to cryostats utilized to produce an inventory of a liquefied cryogen (e.g. helium). The liquefied cryogen can be used to continuously refrigerate and thus operate infra-red detectors, superconducting devices, and the like which require extremely low temperature environments. In the case of helium as the cryogen, temperatures on the order of 4.2.degree. Kelvin (K.) (-268.9.degree. C.) are required to maintain a liquid helium inventory.
2. Description of the Prior Art
For small devices cooled to temperatures on the order of that of liquid helium, miniature cryogenic refrigeration systems including a dewar within which is disposed a heat exchanger containing at the cold end thereof a Joule-Thompson valve or orifice have been used. These devices, as shown in U.S. Pat. No. 3,728,868, utilize a source of high pressure gas which is cooled through the heat exchanger and expanded through the Joule-Thompson valve to provide a small inventory of a liquid cryogen (e.g. helium).
Other prior art systems for producing liquefied cryogens in volume (e.g. helium) are disclosed in U.S. Pat. Nos. 2,458,894 and 3,360,955. Both of the foregoing patents employ one or more expansion engines, together with multi-stage heat exchangers and Joule-Thompson expansion valves to produce liquid helium. In the case of the '894 Patent, the liquid helium is used in heat exchange with air to liquefy air for subsequent fractional distillation to produce oxygen. The '955 apparatus is used for producing low temperatures to cool various types of electrical apparatus including superconducting computers.
One of the problems encountered with all prior art Cryostats is the maintenance of an inventory of liquid helium which has a normal boiling point of 4.2.degree. K. One method of maintaining an inventory of liquid cryogen (e.g. helium) is to provide a continuous supply of gaseous cryogen which is liquefied to replace that which boils off due to heat infiltrating the Cryostat. The venting gaseous cryogen can be collected and recondensed. In order to do this, the vented gas must be recycled to a refrigeration apparatus which, in turn, produces the liquid helium which is reintroduced into the liquid inventory or reservoir. In the case of a superconducting electronic devices (e.g. super conducting magnets, super conducting quantum interference devices and Josephson junction devices) access is required to the liquid inventory so the device being cooled can be placed in the inventory with electrical leads from the liquid inventory to related equipment operating at ambient conditions. Of necessity, this creates an access passage and possible paths of heat infiltration into the Cryostat to promote boil-off of the liquid cryogen with pressure increases inside the Cryostat. It is desirable to have the cryogenic refrigerator disposed in the access passage or within the vacuum space to cool suitable heat stations in the access means to prevent heat infiltration. In order to do this, the refrigerator is preferably disposed within the Cryostat housing. Having the refrigerator in this position, it then becomes necessary to provide means to remove the refrigerator should it have to be serviced, preferably without exposing the liquid cryogen inventory to ambient conditions so as to minimize heat infiltration and cryogen boil-off and to prevent contamination of the cryogen by the ambient atmosphere.
A cryogenic refrigerator ideally suited for this application is manufactured and sold by Air Products and Chemicals, Inc., Allentown, Pennsylvania as a DISPLEX.RTM. Model CS-308 Closed Cycle Helium Refrigeration System. The displacer-expander refrigerator portion of the Model CS-308 is disclosed in the specification of U.S. Pat. No. 3,620,029, which is incorporated herein by reference. A refrigerator of this type has been used successfully to cool such things as sample holders for Mossbauer Spectroscopy by means of non-contact heat exchange as shown in U.S. Pat. No. 3,894,403.